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How much power (if any) do you lose when it gets warm???

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Question - how much power do boat engines lose when the temperature goes up? I know with snowmobiles that a 20 - 30 degree difference in temp can make a huge difference on performance - even on the new FI models that "compensate" for temperature. Reason I ask is I've been very happy with how my motor has been dialed in, ran it yesterday (1st really "warm" day I've been on the water)and it has "lost" a couple hundred RPM's. Everything else - weight, etc - is the same. Motor was running fine but I'm wondering if something else is going on that I should be concerned about. FYI - Motor is a 1999 Mercury 40 4 stroke (carb'ed), boat is a 2001 Alumacraft Navigator 165. Propeller is a Cabellas SS 10 1/2 x 11. Any help is appreciated. Paul

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Your motor is water cooled, from the lake (an unlimited supply of cold coolant), so I wouldn't expect that much of a drop in RPM's due to air temperature.

marine_man

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Cold water is denser than warmer water. That's one of the reasons boats run faster in cold/cool water. As the summer progresses and the lakes warm up your boat will slow down a bit.

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Wait a minute guys, The original question is valid in relation to the temperature of the air and it's corresponding decrease is density with the higher temp. While the water cooling will keep the block at a constant temperature and have a small moderating effect, it's the air density with relation to fixed carb jetting and the subsequent fuel/air mixture that causes the decrease in horsepower output. As the air warms up, it's molecules spread apart, resulting in less weight (mass) per unit of volume. Since a carburetor is a fixed restriction to the volume of air flow (not mass) and it's jets limit fuel flow in the same way providing a constant mass, as the air gets less dense the mixure gets richer.

This change in the stochiometric mixture or air/fuel ratio is what causes the reduction in power output. Ideally, with gasoline as the fuel, the ideal ratio is around 14.7:1 by weight of air to fuel, so you can see as the air becomes less dense the mixture will richen and decrease power. The output curve also drops to the lean side of 14.7:1 with the added problem of increased combustion temperature which can result in burned pistons and other bad stuff. Oxygenated fuel (ethanol) will run better with less air density due to the fuel characteristics. Since the manufacturer usually jets the engine to the rich side to reduce warranty claims, and that jetting most likely done around sea level where the air is most dense to begin with, the rise in air temperature will have a noticeable effect on performance.

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My thoughts exactly.... laugh yeah right.. good write up Hydro. You really know your stuff!!!

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Wait a minute guys, The original question is valid in relation to the temperature of the air and it's corresponding decrease is density with the higher temp. While the water cooling will keep the block at a constant temperature and have a small moderating effect, it's the air density with relation to fixed carb jetting and the subsequent fuel/air mixture that causes the decrease in horsepower output. As the air warms up, it's molecules spread apart, resulting in less weight (mass) per unit of volume. Since a carburetor is a fixed restriction to the volume of air flow (not mass) and it's jets limit fuel flow in the same way providing a constant mass, as the air gets less dense the mixure gets richer.

This change in the stochiometric mixture or air/fuel ratio is what causes the reduction in power output. Ideally, with gasoline as the fuel, the ideal ratio is around 14.7:1 by weight of air to fuel, so you can see as the air becomes less dense the mixture will richen and decrease power. The output curve also drops to the lean side of 14.7:1 with the added problem of increased combustion temperature which can result in burned pistons and other bad stuff. Oxygenated fuel (ethanol) will run better with less air density due to the fuel characteristics. Since the manufacturer usually jets the engine to the rich side to reduce warranty claims, and that jetting most likely done around sea level where the air is most dense to begin with, the rise in air temperature will have a noticeable effect on performance.

Well if you want to get technical. wink

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Good post Hydro... I still think dropping 200 rpm seems like a lot though...

marine_man

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Wait a minute guys, The original question is valid in relation to the temperature of the air and it's corresponding decrease is density with the higher temp. While the water cooling will keep the block at a constant temperature and have a small moderating effect, it's the air density with relation to fixed carb jetting and the subsequent fuel/air mixture that causes the decrease in horsepower output. As the air warms up, it's molecules spread apart, resulting in less weight (mass) per unit of volume. Since a carburetor is a fixed restriction to the volume of air flow (not mass) and it's jets limit fuel flow in the same way providing a constant mass, as the air gets less dense the mixure gets richer.

This change in the stochiometric mixture or air/fuel ratio is what causes the reduction in power output. Ideally, with gasoline as the fuel, the ideal ratio is around 14.7:1 by weight of air to fuel, so you can see as the air becomes less dense the mixture will richen and decrease power. The output curve also drops to the lean side of 14.7:1 with the added problem of increased combustion temperature which can result in burned pistons and other bad stuff. Oxygenated fuel (ethanol) will run better with less air density due to the fuel characteristics. Since the manufacturer usually jets the engine to the rich side to reduce warranty claims, and that jetting most likely done around sea level where the air is most dense to begin with, the rise in air temperature will have a noticeable effect on performance.

This is what I was looking for. I agree the engine operating temp would not change much, I was talking about the different effect warmer AIR would have on the performance. In snowmobiles you jet down (leaner) as it gets warmer to maintain power - if you don't you will lose power. I've never changed the jets in my boat so I am assuming that I am losing power from those 40 degree mornings to the 85 degree days. On top of that if they are jetted rich to begin with you could be leaving lots of power on the table. I guess not 100% definiative in diagnosing my "strange" loss of RPM's but it sounds like I am not thinking totally crazy. Thanks, Paul

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Good post Hydro... I still think dropping 200 rpm seems like a lot though...

marine_man

Maybe, maybe not. Your only talking about a reduction in RPM's of 3.44% which on a 40 Hp motor MAY only mean a redcution in 1.34 hp (not sure of the EXACT relationship between RPM's and HP but I'm sure they're tied together). If you are not jetted right on a snowmobile you could easily lose 10% - 20%. MM - please note I am not saying this IS my problem, I'm just saying when you look at the numbers it really isn't THAT much. Paul

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for my snowmobile it will hit 75 when its -30 when its 30 it will only go 50 and takes a ton of gas.. i think the reason fro sleds dropping power is the wetter snow causing more friction

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for my snowmobile it will hit 75 when its -30 when its 30 it will only go 50 and takes a ton of gas.. i think the reason fro sleds dropping power is the wetter snow causing more friction

Some is friction, A LOT is you do NOT have as much power. You can compare on bare ice (or a grass drag or heck even an asphalt drag strip) - anywhere the friction is relatively "constant". You WILL be faster at -30 versus +30, I guarantee it. Air is denser at -30 which means you can use more gas and still maintain the optimal air to fuel ratio - more gas + denser air = bigger bang!! This is simple physics. If you are not able to go faster at -30, something else is wrong. Paul

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i wonder what a boat motor would do at -30?? it is really fun to ride aournd when its -40 and -60 windchill makes a ski do 550 seem like a rmk 700!!

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Hiya,

I think there might be a bit more to this than just what hydro stated. I would agree 100% with what he stated, but one thing I think may add "a little" to the loss in RPMs is that as was mentioned, high temperatures will force molecules apart...both in air and the water itself. Thus...although minimal, the boat will sit lower in the water...even at planing speeds. This, in effect will result in more "wetted surface" of the boat in the water while at speed, and that in itself will mean lower RPM since the motor has to work harder.

I know with my yamaha 90 2 stroke, I lose about 200 rpms when it gets hot as well...worse when it's humid.

Steve

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